When a photonic crystal layer including a two-dimensional grating has anisotropy, it provides a surface-emitting quantum cascade laser capable of emitting infrared laser light in a direction generally perpendicular to the active layer.
When the two-dimensional grating has the same pattern over the entire surface of the light outgoing surface, the infrared laser light is emitted with a single wavelength.
However, emitting infrared laser light with different wavelengths is required in the case of e.g. generating terahertz waves.